Geological assessment and development of the Sakoshi-Ôdomari gold mine, Hyôgo Prefecture.

Abstract

The Sakoshi-Ôdomari mine is a recently discovered gold mine of quartz vein type, which is located in Akô city, Hyôgo Prefecture, southwestern Japan. The mine is of highly profitable, though it is small in size.
The discovery was made in 1974 by the claim owner of a so-called "pyrophyllite" mine, who happened to assay samples taken from outcrops within the claims that had been neglected as "silicified rocks". Subsequent to the discovery, he offered a joint development programme to Sumitomo Metal Mining Co., Ltd., and the writers were assigned to assess the property by the latter.
The mine area is situated within one of the most famous "pyrophyllite" district in Japan, and mainly comprises widely distributed "pyrophyllitized" welded tuff. Therefore, the region had long been regarded as somewhat barren in metallic mineralization and few people had believed occurrence of a promising gold deposit in the region.
Without such a preconception, the writers carried out detailed geological mapping along with trenching several times, and studied the mode ofe occurrence of ores and wall rock alteration. Their findings were as follows: (1) So-called "pyrophyllitized" rocks in the mine actually consist of kaoline-minerals and sericite, and are, consequently, different from "pyrophyllite" ores in such deposits as Mitsuishi, Hiraki, etc. in the adjacent areas, where the mineralogical pyrophyllite is a major consituent. The assemblage in this mine rather resembles what is commonly observed in a gold deposit than that in a pyrophyllite deposit. (2) Four ore bodies occur en echelon within a strike length of 500 m at the surface. (3) Gold ores are apparently controlled by a quartz porphyry stock. And, therefore, it may be expected that low grade ores at the surface will turn to higher grade at the depths, where the vein approaches to the stock.
From these observations, the writers concluded that a profitable gold deposit could be anticipated to occur, and warranted to participate to the programme.
Based on the conclusion, the company decided to develop the property in 1976. Since then, the mine has being successfully operated up to date. The total production during a period between April 1977 and March 1982 is as follows: Ores mined 34, 400 t at 30.7 g/t Au and 283 g/t Ag.
This paper describes the case study of the geological assessment and development of the mine, as well as general description on geology.